SWMM
A dynamic rainfall-runoff simulation model used for simulation of runoff
and runoff quantity and quality from primarily urban areas.

HEC-RAS
A computer program that models the hydraulics of water flow through
natural rivers and other channels. The program has been developed for
the U.S. Army Corps of Engineers and is available to individuals without
a charge.

Convert
Convert is a free and easy to use unit conversion program that will
convert the most popular units of distance, temperature, volume, time,
speed, mass, power, density, pressure, energy and many others, including
the ability to create custom conversions!